In view of the desirability for a closed papermaking system, rosin emulsion sizing agents for papermaking are in predominant use in recent years. Consequently extensive investigations have been made on dispersing agents for use in preparing the rosin emulsion sizing agents. Conventional dispersants include low-molecular-weight dispersants such as sodium alkylsulfonates, sodium alkylbenzenesulfonates, polyoxyethylene alkyl phenyl ethers and sulfuric acid ester salts of polyoxyethylene alkyl phenyl ethers. However, emulsion sizing agents prepared using the low-molecular-weight dispersant markedly bubble up when used for papermaking and fail to produce a satisfactory sizing effect when used for papermaking at high temperatures or high pH values. Rosin emulsion sizing agents have been developed in which a polymer dispersant is used to obviate the drawbacks of the low-molecular-weight dispersants. These sizing agents are substantially free of bubbling in papermaking, but still unsatisfactory in sizing effect when used for papermaking at high temperatures or high pH values.